Crystal growth habit

Shape Control Based on the Crystal Growth Habit... 

In some cases, new phases that may not be detectable by other methods may be detected optically (Chang et al. 1995). Solid state conversions and their monotropic (Burger et al. 1997) or enantiotropic nature (Henck et al. 2000), or the products of desolvations may be easily recognized (Schinzer et al. 1997). Intimate processes of polymorphic behaviour, such as nucleation, crystal growth, habit transformation, sublimation and properties of the melt (e.g. degradation) may be readily observed and video recorded (de Wet et al. 1998). 

At a higher structural level, the intercrystalline structures or the polymer morphology will be examined. These structures result from the crystal growth habit of the polymer. Most of the published work in this field relates to structures at the molecular level and, in particular, to the polymer chain conformation. The chain conformation is of particular interest, because it is strongly coupled to the electronic structures and the very interesting UV absorption characteristics of these polymers. 

Figure 3.1 Effect of impurities on crystal growth habit. (Reproduced with permission from Addadi et al. 1985.)...

Fig. 5.6. Ice crystal growth habit as a function of temperature and of vapour density excess over the equilibrium value for ice at the temperature in question (after Kobayashi, 1958).

Crystal growth habit can be modified when the relative order of surface energies can be changed or when crystal growth along certain crystallographic directions is selectively hindered by a crystal growth modifier [4]. In the pres-... 

Fig.1 Illustration of the crystal growth process, a Normal crystal habit in the absence of crystal modifiers b the altered crystal growth habit due to the selective/preferential adsorption of crystal modifiers to a specific crystallographic face and c the crystal growth within an confined environment...

Several of the structure types found for the halides are observed for other important monovalent anion systems such as the trihydroxides and trihydrides and derivatives of these structures are observed for a variety of ternary phases (Haschke, 1975c and 1976b). Other monovalent anions, Y, are accommodated in PuBt3- and UCh-type RX3 phases by replacement of the X layers. The resulting RX2Y compositions are structural derivatives of the parent trihalide structures. These observations and the phase equilibria and crystal growth habits of the ternary derivatives support the conclusion that several of the trihalide structures are accurately described by alternation of RXj and X layers, but additional work is clearly necessary. 

Unfortunately the mass accommodation coefficient a is unknown for solid surfaces under stratospheric conditions. It contains information on crystal growth habit and virtually nothing is known about this. MacKenzie... 

Pharmacists should also take a dim view of changes in the particle size, size distribution, or particulate nature of semisolid suspensions. They are the consequence of crystal growth, changes in crystalline habit, or the reversion of the crystalline materials to a more stable polymorphic form. Any crystalline alteration can lead to a pronounced reduction in the drug-delivery capabilities and therapeutic utility of a formulation. Thus, products exhibiting such changes are seriously physically unstable and unusable. 

Keywords Zeolite L, Crystal Habit, Crystal Growth, AFM... 

The habit of pharmaceutical compounds has been used for purposes of identification, although the method can only be reliably used when the crystallization solvent used to generate the test crystals is carefully controlled. Since the faces of a crystal must reflect the internal structure of the solid, the angles between any two faces of a crystal will remain the same even if the crystal growth is accelerated or retarded in one direction or another. Toxicologists have made extensive use of microscopy following multiple recrystallization, and they have developed useful methods for compound identification [5]. 

Davey, R.J., Whiting, M.J.L., (1982). Habit modification of succinic acid crystals grown from different solvents. Journal of Crystal Growth., 58, 304-312. 

Because the rate of growth depends, in a complex way, on temperature, supersaturation, size, habit, system turbulence and so on, there is no simple was of expressing the rate of crystal growth, although, under carefully defined conditions, growth may be expressed as an overall mass deposition rate, RG (kg/m2 s), an overall linear growth rate, Gd(= Ad./At) (m/s) or as a mean linear velocity, // (= Ar/At) (m/s). Here d is some characteristic size of the crystal such as the equivalent aperture size, and r is the radius corresponding to the... 

When three of the oxygens in the tetrahedra are shared (Si O ratio = 2 5), the complex ions that form take on a sheetlike configuration. The sheets can be stacked, and the associated cations are found between the sheets. Micas and clays are sheet-structure minerals with distinctive habits and physical properties, that reflect the planar silicate sheet structure (Fig. 2.1G). These normally platey minerals may also occur with fibrous-growth habits. The special crystal chemistry that produces such a distinctive habit is discussed later. 

The use of tailor made additives holds great promise in the area of crystal growth and morphology control. The routine selection and use of these type of additives will require a fundamental understanding of the mechanism which the additives work on a molecular basis. At the same time, the effect of solvent molecules on the crystal growth process is another related and important problem. In both instances, the relationship between internal aystal structure, aystal growth rate, solvent and impurities are needed to predict the habit of a crystal and thus allow seleaion of the proper conditions and components required to obtain a desired habit... 

In a recent paper (70 Hartman studies the effect of surface relaxation on the habit of corundum and hematite. The habits observed on natural and synthetic crystals of these systems did not agree with calculated relaxed equilibrium or growth habits. Hartman concluded that these observations could be understood by invoking specific solvent adsorption on (111) faces. 

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